1. Field of the Invention
The field of invention relates to a method and system for upgrading petroleum. More specifically, the field relates to a method and system that uses supercritical water to upgrade petroleum.
2. Description of the Related Art
Petroleum is an essential source of energy and petroleum-based chemicals; however, impurities in the petroleum, especially sulfur, require treatment to alleviate potential environmental impacts of consuming the petroleum. Exacerbating the problem is that a greater amount of available petroleum is heavy or sour petroleum, both of which contain greater amount of impurities which must be mitigated before use as fuels.
Traditional processes for “upgrading” the petroleum feedstock include hydrogenative and thermal methods. Hydrogenative methods, which include hydrotreating and hydrocracking, use hydrogen and a hydration catalyst to remove impurities and convert the heavy fraction in the petroleum into a light or middle-range product. The main problems with hydrogenative methods include the amount of hydrogen required and the amount of catalyst, which is relatively easy to deactivate in the presence of sulfur. Thermal methods, which include coking and visbreaking, do not require the addition of external hydrogen or catalyst to facilitate the reaction. Thermal processes do, however, produce coke as a byproduct as well as olefins and diolefins. Thermal processes are also known to be ineffective in removing sulfur and nitrogen heteroatom species.
The application of supercritical water to a petroleum feedstock is known as an effective technique for upgrading hydrocarbons and desulfurization. Although coke formation is suppressed, the supercritical water reaction conditions and residence time required to effectively upgrade the introduced petroleum feedstock can result in pre-coking and coking reactions that result in coating the reactor as well as units and attachments downstream. Increased residence time in the supercritical water reactor increases upgrading conversion and desulfurization; however, the probability of coking and overcracking the hydrocarbons into light gases and olefins also increases. It is desirable to have a method and system that exposes the heavy fraction of the petroleum feedstock to supercritical water such that the heavy fraction breaks down but minimizes the formation of pre-coking materials.
Petroleum-based coke is generated by inter-radical reactions. The inter-radical reactions produce localized dimerization, oligomerization and polymerization reactions between high-carbon hydrocarbons of the petroleum feedstock. Coking is not commonly observed in thermal upgrading treatment processes of light fractions of crude oil, including naphtha, kerosene and diesel. Although not intending to be bound by theory, it is believed that polyaromatics rings of hydrocarbons (for example, asphaltenes) stabilize free-radicals through delocalization. The delocalization effect results in the lifespan of the free-radical on the polyaromatics rings being much longer than the lifespan of a free-radical associated with paraffinic, olefinic or naphthenic compounds. The fraction of the crude oil that possesses the greatest amount of polyaromatic rings is the heavy fraction.
In addition, the hydrocarbons associated with the heavy fraction of the petroleum feedstock are not readily miscible with it even though supercritical water acts as a strong solvent towards hydrocarbons generally. It is desirable to have a system and method that blends supercritical water with the heavy fraction such that upgrading of the heavy fraction is supported.